Bukkit/Bukkit — comment on commit 50939e77697b4b0a9a259a022fe607b8820b5d85
File:  line 0
By EvilSeph:

EGG is for chicken eggs, SPAWNER_EGG is for the new spawner eggs you get in creative.

Debating if I should change EGG to CHICKEN_EGG, though this would be a breaking change. My reasoning behind adding a new separate spawn reason is due to our PlayerEggThrow event (it only applies to chicken eggs being thrown) which clearly showed me that they should be separate.